Polycrystalline silicon, or polysilicon, is a critical raw material for the electronics industry. It is the starting material for production of single-crystal silicon ingots for the semiconductor industry. These ingots are produced by either the Czochralski (CZ) or the Float Zone (FZ) method. The majority of polysilicon produced is consumed in the CZ process.
In the CZ crystal-pulling process, chunks of polysilicon are loaded into a quartz crucible. The chunks of polysilicon are of random size and shape. The crucible is filled and loaded into the CZ furnace. The polysilicon is melted and a single crystal ingot is withdrawn from the melt.
The yield of single crystal silicon is a function of the quantity of molten silicon that can be included within the crucible as well as the quality of the polysilicon added. In order to maximize the initial polysilicon chunk packing density within the crucible, polysilicon chips, granules or short rod pieces may be added to the quartz crucible. After a crystal is pulled from the melt, the crucible may be recharged with additional polysilicon chunk or chips to allow one or more additional pulls from the same crucible.
Recharging a crucible with polysilicon may be accomplished by feeding silicon chips or granules through a feeding apparatus into the crucible. It is desirable that this recharge feed material be small in weight and size in order that problems due to the disturbance of the melt surface by the falling silicon granules be avoided. The small weight and size also allow the polysilicon to be fed to the crucible though valves, tubes or other openings that are of minimal size thus avoiding heat loss and high equipment cost.
Contaminants added to the melt with the polysilicon can impact the yield of single-crystal silicon ingot by inducing defects and/or causing loss of structure. These contaminants can also cause poor device performance in the integrated circuits manufactured from contaminated silicon wafers. Contaminants are found on the polysilicon surfaces or come from the dissolution of the quartz crucible. The polysilicon surface-contamination typically originates from the breaking process used to form chunks. Excessive impacting to form the desirable small silicon chips or granules may result in higher levels of contaminants on the polysilicon surfaces.
The production of polysilicon rods by the pyrolytic decomposition of a gaseous silicon compound, such as silane or a chlorosilane, on a suitable filament substrate is a well-known process. The process comprises:                a) An even number of electrodes are attached to a base plate, each electrode can have a starting filament (starter rod) attached.        b) The filaments are joined in pairs by a connecting bridge. Each bridge is a piece of starter rod material and is joined to two starting filaments. Each set of two filaments and a bridge thus is an inverted, generally U-shaped member, commonly referred to as a hairpin. For each hairpin assembly, an electrical pathway is formed between a pair of electrodes within the reactor. An electrical potential applied to the electrodes can thus supply current required to heat the attached hairpin resistively.        c) The hairpins are contained in a bell jar enclosure that mates with the base plate to define a batch reactor allowing operation under vacuum or positive pressure conditions.        d) A gaseous silicon precursor compound of the desired semiconductor material and other gases, as necessary, are fed into the reactor.        e) The hairpins are electrically heated to a temperature sufficient to effect decomposition of the gaseous precursor compound and simultaneous deposition of the semiconductor material onto the hairpins, thereby producing generally U-shaped polysilicon rods of substantial diameter. The rods are generally circular in cross-section, except at the corners where the filaments are joined to the connecting bridges.        f) Any by-product gases and unreacted precursor compounds are exhausted from the reactor.        
The principles of design of present state of the art reactors for the pyrolysis of silane and chlorosilanes are set forth in, for example, U.S. Pat. Nos. 4,150,168; 4,179,530; 4,724,160; 4,826,668; and 6,365,225.
The temperature of the decomposition process is carefully controlled in order to maintain the rod surface relatively smooth while maintaining reasonable reaction rates and conversion of silicon precursor compounds to silicon. The surface temperature controls the roughness of the deposited layer of polysilicon as well as the reaction kinetics. Lower surface temperatures result in smoother rods because the deposition is uniform over the rod surface. However, lower temperatures slow the reaction kinetics resulting in a reduced deposition rate, as do lower concentrations of silicon in the reactor environment.
Higher surface temperatures increase deposition rate, but they also initiate selective deposition onto surface mounds. Under normal conditions, rod temperature is raised until the rod surface is covered with small mounds whose edges are fused with neighboring mounds and free of void spaces. This condition represents the fastest deposition rate (fastest reaction kinetic rate) while maintaining a rod surface free of void spaces. Such mounds are integrally attached to the rod and have an aspect ratio, as defined below, that is less than one (<1), typically 0.5 or less.
As chemical vapor deposition proceeds, the diameter of the silicon rod increases. Power is increased to maintain the silicon rod at an appropriate surface temperature. The total energy consumed to produce a unit mass of polysilicon depends upon the rod surface temperature, final rod diameter (power applied) and the rate at which silicon precursor gas decomposes to form silicon. If rods can reach the target diameter faster (due to higher reaction kinetics and higher deposition rates), the energy required to maintain the rods at temperature is reduced, decreasing the overall energy requirement to produce a unit mass of polysilicon.
The processing of polysilicon rods into chunks or chips is either a mechanical or a thermo-mechanical process. For Czochralski crystal growth, silicon rods are broken into chunks or chips using mechanical devices such as crushers, mills or hammers. In addition, thermally stressing rods aids in the breaking process. Typically, rods are broken into irregularly shaped chunks up to about 100 mm in size.
Mechanical breaking devices add contamination to the polysilicon pieces due to physical impacting. In order to produce the desirable smaller chips (10 to 30 mm), additional impacting is required, which impacting leads to higher levels of silicon surface contamination. This surface contamination, if great enough, must be removed by cleaning or acid etching or it can impact the yield of the single crystal silicon ingot. These cleaning and etching systems add cost to the polysilicon manufacturing process.